The present invention relates to a seamless, ring-shaped brazing material for brazing the same or different types of nonferrous metal members such as aluminum pipes, copper pipes, etc. and to a method for producing same.
Known as a method for achieving gas-tight contact of nonferrous metal members such as aluminum pipes is a method which comprises having a brazing alloy penetrate into a gap between contact surfaces of the metal members to be connected. Penetration by capillary effect occurs, when the brazing metal placed between or near the contact surfaces of the aluminum metal members is heated at a temperature equal to or higher than the melting point of the brazing alloy and lower than the melting point of aluminum. In general, it is desirable that the melting point of the brazing alloy is at least 30-40.degree. C. lower than those of the metal members to be connected. Melting the brazing alloy is achieved, for instance, by heating in an inactive gas atmosphere, high-frequency heating, gas torching in the air, etc.
To remove oxides from the contact surfaces of the nonferrous metal members and increase the affinity of the nonferrous metal members for the brazing metal, a flux is applied to the contact surfaces of the nonferrous metal members. The flux is a substance, which is decomposed when melted at a brazing temperature, thereby reacting with the oxides on the contact surfaces of the metal members, resulting in their removal. Generally, the fluxes such as fluorinated potassium aluminates, chlorinated potassium aluminates, etc. are used for the brazing of aluminum.
Because many steps are needed to carry out brazing by applying a flux to a contact surface, placing a brazing material thereon and heating it by a torch, etc., the use of a so-called clad plate having a flux sandwiched by the brazing alloy was proposed to simplify the brazing process. Though the clad plate is effective for particular cases, for example, for the connection of metal members having complicated shapes, the clad plate is disadvantageous in that it is expensive. Further, because the brazing material is brought into contact with the entire surface of a substrate to be connected in the case of using the clad-type brazing material, the brazing material is likely to erode the substrate, resulting in the deformation of the substrate.
Also known is a brazing method comprising applying a paste which is a uniform mixture of brazing alloy powder and a flux powder in an organic solvent such as isopropyl alcohol to contact surfaces of metal members, drying the paste, and melting it by heating to connect both metal members. A proposed composition of the paste is a mixture of aluminum alloy powder, flux powder and a binder such as an acrylic resin in a solvent.
U.S. Pat. No. 3,971,501 discloses a brazing method comprising applying a mixture of brazing alloy powder and flux powder to surfaces of aluminum members to be connected and heating the aluminum surfaces to a brazing temperature after drying. U.S. Pat. No. 5,100,048 discloses a method comprising applying a composition of silicon metal powder and flux powder to contact surfaces of aluminum members to be connected and heating it so that an eutectic alloy is formed from the silicon metal powder and the aluminum member, thereby carrying out brazing. Though these methods are useful for connecting members having broad contact surfaces, they are disadvantageous in cases of connecting metal members having narrow contact surfaces such as pipes, because brazing materials are not sufficiently applied to the contact surfaces.
Recently proposed is the production of a rod-shaped, flux-containing brazing material by mixing of aluminum alloy powder and flux powder, hot-pressing the resultant mixture and forming it into an integral rod (Japanese Patent No. 2,584,926). However, the flux-containing brazing material is so brittle that it has difficulty in being bent into a ring.
WO 94/17941 discloses a method for producing a flux-coated aluminum alloy brazing material comprising ejecting a molten aluminum alloy by a gas atomization method or a rotating-disc method while spraying flux powder to the resultant brazing powder at the site of ejection. Coating the flux is done at the same moment when the brazing material is solidified. In this method, however, it is difficult to keep the combination ratio of the brazing alloy to the flux uniform and the compositions of products less variable.
In a case where metal pipe members such as aluminum pipes are connected by brazing with a composite brazing material comprising a brazing metal and a flux tuiformly dispersed therein, the composite brazing material should be formed into a ring. It has been found, however, that a brazing wire formed by the hot extrusion of a brazing metal and a flux is too brittle to be bent to a ring, and that a remaining stress due to bending causes the resultant ring to open during the heating for brazing. Also, since both ends of the ring tend to be first melted, it is difficult to achieve uniform brazing.
In the conventional brazing materials, a complex compound containing K, Al and F is used as a flux. The complex compound, however, is too expensive, because it is produced by melting and mixing a flux-forming material powder such as AlF.sub.3 powder, KF powder, Al.sub.2 O.sub.3 powder and CsF powder.